This invention relates generally to thin film resonators and more specifically to a method and apparatus for adjusting the resonant frequency of a thin film resonator.
As is known in the art, a quartz crystal exhibits mechanical deformation when a voltage is applied across opposing faces of the crystal. Conversely, if the crystal is mechanically strained, a voltage differential is produced across the crystal faces. This phenonenom is referred to as the piezoelectric effect. Electrically, the quartz crystal operates as a parallel inductor-capacitor tuned circuit. Typically, crystal filters have an extremely high Q (a figure of merit defining the filter bandwidth) and excellent frequency stability.
Piezoelectric crystals formed on a semiconductor substrate are referred to as thin film resonators or thin film acoustic devices. These thin film devices are electro-mechanical resonators whose resonant frequency is determined by the thickness and acoustic properties of the piezoelectric film. Typically, the devices resonate in the radio frequency to microwave range, for example from about 0.5 gigahertz (GHz) to about 5 GHz in response to an electrical signal applied across, opposing surfaces of the piezoelectric film. The film is conventionally comprised of piezoelectric crystalline material, such as zinc oxide, aluminum nitride, or other crystalline materials that exhibit the piezoelectric effect. The fundamental resonant frequency of the piezoelectric material is determined by dividing the acoustic velocity of the film by twice the film thickness, i.e., fr=v/2 t. When subjected to an alternating electrical field having a fundamental frequency and harmonics, the piezoelectric film will mechanically vibrate if one of the frequency components is equal to the piezoelectric resonant frequency.
Since the resonant frequency of the piezoelectric material is determined by the thickness and acoustic properties of the piezoelectric film, it is established at the time of fabrication and not tunable thereafter. Since manufacturing process variations can influence the film thickness, considerable effort is expended to rigidly control the manufacturing processes to maintain tight control over the thickness. For example, to achieve the desired resonant frequency requirements, it may be necessary to control the film thickness with a tolerance of less than about 0.1% of the thickness. For example, in one embodiment, deposition of a 1000 nanometer film must be achieved with an accuracy of 0.1%, which is 1 nanometer. Current semiconductor manufacturing processes for thin film deposition are difficult to control beyond about 1% of the film thickness.
The present invention teaches a method for tuning the resonant frequency of a thin film resonator disposed on a semiconductor substrate. Two electrodes are also disposed on the substrate and a voltage applied between them to create a deformation in the substrate. This deformation is also communicated to the thin film resonator, thereby modifying the resonant frequency thereof.